This invention relates to a needle thread feeding mechanism which positively feeds the required length of needle thread per stitching, such that well-balanced stitchings are obtained.
Referring to FIG. 8, one conventional type of thread feeding mechanism will be explained. FIG. 8 is taken from U.S. Pat. No. 4,566,396 entitled, "Thread Feed Mechanism in Sewing Machine." In such a device, a thread T is arranged to pass through two solenoids 56, 57. Solenoids 56, 57 are adapted to hold the thread T when energized.
Between the solenoids 56, 57 is a paying-out lever 62. The lever lifts the thread T in association with main shaft rotation such that the thread T is fed when the first solenoid 56 is off and the second solenoid 57 is on. Solenoids 56, 57 act alternately such that when one is off, the other is on.
A roller 65 is connected to a pulse generator (not shown) such that thread length is detectable as it is fed. When the required length of thread T is detected, the first solenoid 56 turns on and the second solenoid 57 turns off such that the thread T is fed to the needle.
The motion of the paying-out lever 62, take up lever 55, and needle 52 are synchronized. Timing for the on-off action of solenoids 56 and 57 is set at 85 degrees of the main shaft phase angle, the phase angle at which thread slackening by the take-up lever begins. The thread T is fed positively and intermitently by the required pitch length by repeating the above-mentioned on-off control of solenoids 56, 57, as set forth in U.S. Pat. No. 4,566,396.
In such prior art, thread passage is very complicated because there are many turnbacks and right-turns and passage through two solenoids. Simpler passage is desirable.
Another conventional way of feeding thread is through the use of a step motor to control the required length of feed But where the stitching speed is high, the step motor may not work accurately in response to the pulse signals. In addition, a step motor adds considerable cost.